Method and system for laying underground a continuous elongated member in a bed of a body of water

ABSTRACT

A method of underground laying a continuous elongated member in a bed of a body of water, wherein the continuous elongated member lies on the bed of the body of water along a given path; the method including the steps of:
         fragmenting a soil mass in the bed along the given path and under the continuous elongated member, so as to form in the bed two scarp slopes bounding the fragmented soil mass by two soil masses susceptible to slide;   advancing two supporting walls along the given path in an advancing direction, along the respective two scarp slopes, and transferring the fragmented soil mass between the two supporting walls, so as to promote sinking of the continuous elongated member between the two supporting walls.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. Nationalization of PCT InternationalApplication No. PCT/IB2009/006734 filed 3 Sep. 2009, which claimspriority to Italian Patent Application No. MI2008A001586 filed 4 Sep.2008, the entireties of both of the foregoing applications areincorporated herein by reference.

TECHNICAL FIELD

Embodiments of the present invention relates to a method of undergroundlaying a continuous elongated member, such as an underwater pipeline,cable, umbilical, pipe and/or cable bundle, in the bed of a body ofwater.

BACKGROUND ART

In-bed laying an underwater pipeline normally comprises laying thepipeline along a given path on the bed of the body of water; fragmentinga soil mass along the path to a given depth; digging a trench orgenerally removing the fragmented soil mass; and possibly burying thepipeline.

More specifically, currently used known techniques comprise removing thefragmented soil mass to form a trench in the bed of the body of water;and laying the pipeline directly into the trench. The pipeline may laterbe covered over with the removed soil mass to fill in the trench andbury the pipeline.

Underwater pipelines carrying hydrocarbons are normally laid completelyor partly underground for various reasons, some of which are discussedbelow. Underwater pipelines are normally laid underground close to shoreapproaches and in relatively shallow water, to protect them from damageby blunt objects, such as anchors or fishing nets, and are sometimeslaid underground to protect them from natural agents, such as wavemotion and currents, which may result in severe stress. That is, when apipeline is laid on the bed of a body of water, it may span twosupporting areas of the bed, i.e. a portion of the pipeline may beraised off the bed; in which case, the pipeline is particularly exposedto, and offers little resistance to the movements induced by, wavemotion and currents. Underground laying may also be required for reasonsof thermal instability, which result in deformation (upheaval/lateralbuckling) of the pipeline, or to protect the pipeline from themechanical action of ice, which, in particularly shallow water, mayresult in scouring of the bed.

To avoid damage, the pipeline often need simply be laid at the bottom ofa suitably deep trench dug before laying (pre-trenching) or more oftenafter laying the pipeline (post-trenching). At times, the protectionafforded by the trench and eventual natural backfilling of the trench isnot enough, and the pipeline must be buried using the fragmented soilmass removed from the trench, or any available soil mass alongside thetrench.

The depth of the trench is normally such that the top line of thepipeline is roughly a meter below the surface of the bed, though severeenvironmental conditions may sometimes call for deeper trenches (ofseveral meters). Trenching and backfilling are performed using diggingequipment, and post-trenching (with the pipeline already laid on thebed) is the normal practice, to dig and backfill the trench in one go.

One method of in-bed laying underwater pipelines is described in PatentApplication WO 2005/005736. This is a post-trenching method comprisingthe steps of fragmenting a soil mass in the bed to open the way; anddrawing along the opening a huge plough, to form a trench, and verticalsupporting walls connected to the plough and which respectively supporttwo opposite soil masses bounded by two substantially vertical scarpslopes.

The above method has the drawback of being highly energy-intensive, duepartly to the plough, and partly to friction between the supportingwalls and the two soil masses. And energy consumption increasesexponentially alongside an increase in trench depth.

Another method of in-bed laying underwater pipelines is described inPatent Application WO 2004/016366, which proposes fragmenting a soilmass in the bed, and removing the fragmented soil mass using a dredgingunit on board a support vessel. That is, the fragmented soil mass isfirst sucked up from the bed along a dredging path up onto the supportvessel, and then dumped back into the trench.

This method is also highly energy-intensive to draw the fragmented soilmass up onto the support vessel. Moreover, the scarp slopes aresusceptible to slide; the method is unsuitable for in-depth layingunderwater pipelines; and, in the event of slide, the pumps and conduitsare called on to remove additional fragmented soil masses, thus furtherincreasing energy consumption.

SUMMARY

One or more embodiments of the present invention provide a method ofunderground laying an underwater pipeline in the bed of a body of water,designed to eliminate the drawbacks of the known art.

One or more embodiments of the present invention provide a methodenabling easy in-depth laying of underwater pipelines in the bed of abody of water.

According to an embodiment of the present invention, there is provided amethod of underground laying a continuous elongated member in a bed of abody of water, wherein the continuous elongated member lies on the bedof the body of water along a given path; the method including the stepsof:

-   -   fragmenting a soil mass in the bed along the given path and        under the continuous elongated member, so as to form in the bed        two scarp slopes bounding the fragmented soil mass by two soil        masses susceptible to slide;    -   advancing two supporting walls, along the given path in an        advancing direction, along the respective two scarp slopes; and    -   transferring the fragmented soil mass between the two supporting        walls, so as to promote sinking of the continuous elongated        member between the two supporting walls.

Embodiments of the present invention provide for greatly reducing energyconsumption by only removing the fragmented soil mass between thesupporting walls preventing slide of the soil masses defined by thescarp slopes, thus enabling in-depth laying with the removal of only asmall fragmented soil mass in relation to depth.

Another embodiment of the present invention provides a system forunderground laying a continuous elongated member in the bed of a body ofwater.

According to an embodiment of the present invention, there is provided asystem for underground laying a continuous elongated member in a bed ofa body of water, wherein the continuous elongated member extends on thebed along a given path: the system comprising an underwater vehiclecomprising a work assembly which is set into the bed and comprises:

-   -   a fragmenting unit for fragmenting a soil mass in the bed along        the given path and under the continuous elongated member, so as        to form in the bed two scarp slopes bounding the fragmented soil        mass by two soil masses susceptible to slide;    -   a sustaining unit comprising two supporting walls which are        advanced, along the given path in an advancing direction, along        the respective two scarp slopes; and    -   means for transferring the fragmented soil mass between the two        supporting walls, so as to promote sinking of the continuous        elongated member between the two supporting walls.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of non-limiting embodiments of the present invention will bedescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1 shows a partly sectioned side view, with parts removed forclarity, of a system for underground laying an underwater pipeline inthe bed of a body of water;

FIG. 2 shows a cross section of the bed when digging a trench in whichto lay the underwater pipeline;

FIG. 3 shows an isometric view, with parts removed for clarity, of anunderwater vehicle of the FIG. 1 system;

FIG. 4 shows a plan view, with parts removed for clarity, of the FIG. 3underwater vehicle;

FIG. 5 shows a larger-scale front view, with parts removed for clarity,of the FIG. 3 underwater vehicle;

FIG. 6 shows an isometric view, with parts removed for clarity, of theFIG. 3 underwater vehicle in another configuration;

FIG. 7 shows a partly sectioned isometric view, with parts removed forclarity, of the FIG. 3 underwater vehicle;

FIG. 8 shows a side section, with parts removed for clarity, of the FIG.3 underwater vehicle;

FIG. 9 shows a larger-scale isometric view, with parts removed forclarity, of a detail of the FIG. 3 underwater vehicle;

FIG. 10 shows a front section, with parts removed for clarity, of adetail of the FIG. 3 underwater vehicle;

FIG. 11 shows a larger-scale side view, with parts removed for clarity,of a detail of the FIG. 3 vehicle;

FIG. 12 shows a larger-scale section, with parts removed for clarity, ofa detail of the FIG. 11.

DETAILED DESCRIPTION Underwater Pipeline Underground Laying System

Number 1 in FIG. 1 indicates as a whole a system for underground layingunderwater pipelines in a bed 2 of a body of water 3 of level SL.

In the following description, the term “body of water” is intended tomean any stretch of water, such as sea, ocean, lake, etc., and the term“bed” is intended to mean the concave layer of the earth's crustcontaining the mass of water in the body.

Underground laying system 1 provides for underground laying anunderwater pipeline 4, which has an axis A1, extends along a given pathP on bed 2, and has been laid beforehand by a known laying vessel notshown in the drawings. Underground laying system 1 comprises a supportvessel 5; and a convoy 6 comprising a number of underwater vehicles 7,8, 9, 10 advanced in an advancing direction D1 along path P.

Though the present description refers specifically to an underwaterpipeline, underground laying system 1 provides for underground layingcontinuous elongated members of all types, such as cables, umbilicals,pipe and/or cable bundles, not shown in the drawings.

Underwater vehicles 7, 8, 9, 10 are guided along path P by supportvessel 5. More specifically, support vessel 5 serves to guide vehicles7, 8, 9, 10 along path P, and to supply vehicles 7, 8, 9, 10 withelectric power, control signals, compressed air, hydraulic power, etc.,so each underwater vehicle 7, 8, 9, 10 is connected to support vessel 5by a cable bundle 11.

Each vehicle 7, 8, 9, 10 is designed to:

-   -   fragment a respective soil layer of bed 2 to form two soil        masses 12 bounded by respective opposite, substantially vertical        scarp slopes 13, as shown clearly in FIG. 2, and a fragmented        soil mass 14 between the two scarp slopes 13;    -   support soil masses 12 along scarp slopes 13 (FIG. 2);    -   transfer the fragmented soil mass 14 between the two opposite        scarp slopes 13 (FIG. 2);    -   guide pipeline 4; and    -   bury pipeline 4 with the removed fragmented soil mass 14.

Underwater vehicles 7, 8, 9, 10 are kept close together to seamlesslysink pipeline 4.

In the FIG. 1 example, underwater vehicle 10 performs no fragmentingfunction.

The fragmented soil mass 14 is bounded at the bottom by bottom faces 15,16, 17 increasing gradually in depth in the opposite direction todirection D1.

In other words, underwater vehicles 7, 8, 9, 10 dig a trench 18, on thebottom face 17 of which pipeline 4 is laid and covered with fragmentedsoil mass 14.

With reference to FIG. 2, for the purpose of this description, the term“scarp slope” is intended to mean a surface connecting rock formations,sediment or terrains at different heights, and, in the example shown,scarp slopes 13 are substantially vertical.

Depending on the depth of trench 18 and the nature of soil mass 12, soilmasses 12 bounded by respective scarp slopes 13 must be supported toprevent soil masses 12 from sliding.

For example, a soil mass of granular material, such as sand or gravel,tends to settle into a surface (natural slope) at a given angle, knownas natural slope angle, to the horizontal.

If bed 2 is made solely of cohesive rock, on the other hand, there ispractically no risk of soil masses 12 sliding at scarp slopes 13.Nevertheless, underground laying system 1 (FIG. 1) is designed to copewith any type of problem, regardless of the geological structure of bed2.

Underwater Vehicles

The following is a detailed description of underwater vehicle 9, withreference to FIGS. 3-10. Underwater vehicles 7, 8, 10 in FIG. 1 are notdescribed in detail, but are structurally similar to underwater vehicle9, from which they differ solely as regards the size of certaincomponent parts. Accordingly, the reference numbers used with referenceto underwater vehicle 9 also apply to corresponding parts of underwatervehicles 7, 8, 10 in FIG. 1.

In FIG. 3, underwater vehicle 9 extends along an axis A2, and comprisesa work assembly 19 which is set into bed 2; two drive assemblies 20which rest on bed 2 and advance work assembly 19 in direction Dl (FIG.1); and two connecting assemblies 21, each for connecting a respectivedrive assembly 20 to work assembly 19, and for adjusting the relativepositions of drive assemblies 20 and work assembly 19.

Work assembly 19 comprises a supporting frame 22; a sustaining unit 23;a fragmenting unit 24; a dredging unit 25; and an auxiliary dredgingunit 26.

Supporting frame 22 substantially comprises a number of beams, each ofwhich is inverted-U-shaped, as shown more clearly in FIG. 7.

Sustaining unit 23 comprises two opposite supporting walls 27 fixed toframe 22 and parallel to axis A2. As shown more clearly in FIG. 10, eachsupporting wall 27 comprises a base structure 28; a number of panels 29connected elastically, preferably by rubber fasteners, to base structure28; and a number of actuators 30 for inducing vibration in panels 29,preferably in a vertical direction D2 crosswise to axis A2 and parallelto supporting walls 27.

With reference to FIG. 5, fragmenting init 24 comprises a number ofvertical cutters 31 for fragmenting a soil mass cross section of a widthsubstantially equal to the distance between opposite walls 27.Fragmenting unit 24 also comprises two arms 32, each of which supportshalf the number of cutters 31 and rotates, with respect to frame 22,about a vertical axis A3 (parallel to supporting walls 27) to setcutters 31 to a work position, in which arms 32 are perpendicular tosupporting walls 27 and cutters 31 connect opposite supporting walls 27,and a rest position, in which arms 32 are parallel to supporting walls27, so the pipeline can be placed between the two arms 32 and respectivecutters 31.

Dredging unit 25 comprises two dredging devices 33. As shown moreclearly in FIG. 8, each dredging device 33 is fitted to underwatervehicle 9 and located at least partly between walls 27. In the exampleshown, each dredging device 33 comprises a suction conduit 34 having asuction port 35 located at the bottom of supporting wall 27 and, in use,under pipeline 4 (FIG. 1); a delivery hose 36 for unloading thefragmented soil mass 14 downstream from convoy 6 (FIG. 1); and a pump 37between suction conduit 34 and hose 36.

With reference to FIG. 7, auxiliary dredging unit 26 comprises two pumps38 (only one shown in FIG. 7) located on opposite sides of sustainingunit 23; and a number of conduits 39 extending between and directly oversupporting walls 27. Each conduit 39 comprises two branches 40respectively adjacent to the inner faces of opposite supporting walls27; and a header 41 communicating with both branches 40 and having anoutlet port 42. Each branch 40 comprises a suction port 43 located closeto the bottom of respective supporting wall 27 and facing the oppositesupporting wall 27.

Pumps 38 are connected to each branch 40 by a respective hose 44 whichgenerates an upward jet in respective branch 40, so that each conduit 39acts as an ejector pump between suction ports 43 and outlet port 42.

With reference to FIG. 4, each drive assembly 20 comprises a supportingbody 45; and a powered track 46 looped about supporting body 45 andmovable about supporting body 45 by known means not shown in thedrawings. Supporting body 45 is at least partly hollow, and comprises acontrol device 47 in turn comprising valves and a pump (not defined indetail), and a pipe 48 connected to the laying vessel to feed/expel airto/from body 45 and so alter the buoyancy of drive assembly 20 andunderwater vehicle 9 as a whole. In other words, supporting body 45 is avariable-buoyancy body.

Each connecting assembly 21 comprises two articulated joints 49, each ofwhich comprises a bracket 50 fitted to supporting body 45 to rotateabout an axis A4; an arm 51 hinged to bracket 50; and an actuator 52, inparticular a hydraulic cylinder, hinged to bracket 50 and arm 51 toform, with bracket 50 and arm 51, a variable-configuration triangle. Arm51 is in turn hinged to a connecting member 53 fitted to work assembly19 as shown in FIG. 9.

With reference to FIG. 9, connecting member 53 comprises a fork 54; anda dove-tail prismatic body 55 with a threaded central hole.

With reference to FIG. 6, connecting assembly 21 also comprises fourtracks 56 which, in the example shown, are grooves extending alongsupporting walls 27 in direction D2 (FIG. 1). More specifically, eachsupporting wall 27 has two tracks 56; and two actuators 57, each locatedat a respective track 56 and connected to connecting member 53 to moveconnecting member 53 (FIG. 9) with respect to supporting wall 27.

With reference to FIG. 9, each track 56 has a seat having a dove-tailedcross section and engaged in sliding manner by prismatic body 55.

With reference to FIG. 8, each actuator 57 is fitted to frame 22, andcomprises an electric motor 58; and a threaded bar 59 housed in the seatof track 56 and engaging the threaded hole in prismatic body 55 so as toform, with prismatic body 55, a screw-nut screw mechanism.

With reference to FIG. 6, each connecting assembly 21 comprises two towbars 60 fitted to a respective pair of connecting members 53 andadjacent to a respective supporting wall 27. Each tow bar 60 ofunderwater vehicle 9 is connected to the respective tow bars of adjacentunderwater vehicles 8 and 10, as shown in FIG. 1.

With reference to FIG. 1, hoses 36 of dredging devices 33 all extenddownstream from the last underwater vehicle 10 in conveyor 6, and haveoutlet ports 61 located over pipeline 4, so the material removed bydredging devices 33 is fed back into trench 18 once pipeline 4 is sunk.

With reference to FIG. 8, the work assembly also comprises a number ofcarriages 62 fitted to frame 22 and located between supporting walls 27to push pipeline 4 downwards and so aid in sinking pipeline 4.

With reference to FIG. 11, each panel 29 has an outer face 63; an innerface 64 (FIG. 12); and vertical ribs 65 and horizontal ribs 66 forstiffening panel 29.

Panel 29 is equipped with a lubricating device 67 for forming a waterfilm along outer face 63 of panel 29, and which comprises a number ofnozzles 68 equally spaced along outer face 63; conduits 69 at verticalribs 65 (FIG. 12); and a pump (not shown) connected to conduits 69 byhoses 70 (FIG. 12).

Nozzles 68 are housed in recesses 71 in panel 29, so as not to projectfrom outer face 63.

With reference to FIG. 12, each nozzle 68 is oriented to emit a jet at a20° angle with respect to outer face 63 and in the opposite direction toadvancing direction D1 (FIG. 11).

With reference to FIG. 11, the size of the jets and the number ofnozzles are selected to cover the whole of outer face 63 with a film ofwater and so reduce friction between panel 29 and scarp slope 13 (FIG.2).

Operation of system 1 will be clear from the above description.

Advantages

In addition to the energy-saving advantages already mentioned, thefragmented soil mass is removed by dredging unit 25 and auxiliarydredging unit 26. In many applications, dredging unit 25 is unable toremove all the fragmented soil mass 14 on its own, so the rest offragmented soil mass 14 is removed by auxiliary dredging unit 26.

Soil masses 12 are prevented from sliding at the fragmenting, removal,and sinking stages, by being confined by supporting walls 27; andfriction between supporting walls 27 and soil masses 12 is greatlyreduced by vibrating panels 29 contacting soil masses 12 along scarpslopes 13.

Underwater vehicles 7, 8, 9, 10 are highly versatile, and can adjust theposition of work assembly 19 with respect to drive assemblies 20 andhence the depth of the work assembly in bed 2.

The distance between drive assemblies 20 and work assembly 19 can alsobe adjusted. For example, in sandy beds, it is best to keep driveassemblies 20 as far away as possible from work assembly 19, to preventthe weight of drive assemblies 20 from inducing slide of soil masses 12and so further increasing friction between soil masses 12 and supportingwalls 27.

Conversely, in rocky beds, where the above drawback does not apply, itis best to keep drive assemblies 20 as close as possible to workassembly 19, so as to provide greater forward thrust to fragmenting unit24, which encounters considerable resistance in rocky terrain.

Because of the play between each track 56 and respective connectingmember 53 and independent actuators 57, work assembly 19 can be tiltedslightly with respect to drive assemblies 20.

Independent actuators 57 enable the two drive assemblies 20 to be set totwo different heights with respect to work assembly 19, and therefore tooperate at two different levels on either side of work assembly 19,while keeping work assembly 19 vertical.

Because cutters 31 can be set to a work position and a rest position,underwater vehicles 7, 8, 9, 10 can be withdrawn from the trench withoutinterfering with pipeline 4 being sunk.

The above feature enables one or more underwater vehicles 7, 8, 9,10—for example, underwater vehicle 10 in FIG. 1—to be used solely forremoval, support and sinking work.

Removal and setup of underwater vehicles 7, 8, 9, 10 are also madeeasier by the variable buoyancy of supporting bodies 45.

Clearly, changes may be made to the embodiment of the present inventionas described herein without, however, departing from the scope of theaccompanying Claims.

1. A method of underground laying a continuous elongated member in a bedof a body of water, wherein the continuous elongated member lies on thebed of the body of water along a given path, the method comprising:fragmenting a soil mass in the bed along the given path and under thecontinuous elongated member, so as to form in the bed two scarp slopesbounding the fragmented soil mass two soil masses susceptible to slide;advancing two supporting walls, along the given path in an advancingdirection, along the respective two scarp slopes; and transferring thefragmented soil mass between the two supporting walls, so as to promotesinking of the continuous elongated member between the two supportingwalls.
 2. The method according to claim 1, further comprising vibratingat least a panel of each supporting wall; the panel contacting arespective soil mass along the respective scarp slope.
 3. The methodaccording to claim 2, wherein the panel of a respective supporting wallis vibrated in a direction crosswise to the given path in asubstantially vertical direction.
 4. The method according to claim 1,further comprising lubricating with a water film the outer faces of thesupporting walls contacting the respective scarp slopes.
 5. The methodaccording to claim 1, wherein the step of advancing said two supportingwalls is performed by at least one underwater vehicle comprising driveassemblies movable on the bed; and a work assembly which is set into thebed and includes said supporting walls.
 6. The method according to claim5, further comprising adjusting the relative position of the workassembly and the drive assemblies via two connecting assemblies, eachlocated between the work assembly and a respective drive assembly. 7.The method according to claim 6, further comprising angling the workassembly with respect to the drive assemblies via the connectingassemblies.
 8. The method according to claim 6, further comprisingadjusting the depth of the work assembly in the bed of the body of watervia the two connecting assemblies.
 9. The method according to claim 6,further comprising adjusting the distance between each drive assemblyand the work assembly via the two connecting assemblies.
 10. The methodaccording to claim 5, further comprising transferring the fragmentedsoil mass via at least one dredging unit comprising a suction portlocated between the two supporting walls, and an outlet port locatedover the continuous elongated member.
 11. The method according to claim10, further comprising supporting the dredging unit via the underwatervehicle.
 12. The method according to claim 1, wherein the step offragmenting the soil mass is performed by a fragmenting unit fordefining a given fragmented cross-section; the width of the fragmentedcross-section being substantially equal to the distance between said twosupporting walls.
 13. The method according to claim 12, furthercomprising setting the fragmenting unit in a work position and a restposition.
 14. The method according to claim 1, further comprisingpushing the continuous elongated member downwards between the twosupporting walls.
 15. A system for underground laying a continuouselongated member in a bed of a body of water, wherein the continuouselongated member extends on the bed along a given path, the systemcomprising: an underwater vehicle comprising a work assembly which isset into the bed, the underwater vehicle including: a fragmenting unitfor fragmenting a soil mass in the bed along the given path and underthe continuous elongated member, so as to form in the bed two scarpslopes bounding the fragmented soil mass by two soil masses susceptibleto slide; a sustaining unit comprising two opposite supporting wallswhich are advanced, along the given path in an advancing direction,along the respective two scarp slopes; and means for transferring thefragmented soil mass between the two supporting walls, so as to promotesinking of the continuous elongated member between the two supportingwalls.
 16. The system according to claim 15, wherein each supportingwall comprises at least a base structure, a vibrating panel connected tothe base structure, and means for inducing vibration of the panel; eachpanel being positioned contacting a respective soil mass along therespective scarp slope.
 17. The system according to claim 16, whereinthe supporting wall comprises vibrating means for inducing vibration ofthe panel in a direction crosswise to the given path in a substantiallyvertical direction.
 18. The system according to claim 15, furthercomprising a lubricating device for lubricating with a water film theouter faces of the supporting walls contacting the respective scarpslopes.
 19. The system according to claim 15, wherein the underwatervehicle comprises two drive assemblies movable on the bed and connectedto the work assembly.
 20. The system according to claim 19, wherein eachdrive assembly comprises a supporting body of adjustable buoyancy. 21.The system according to claim 20, wherein each drive assembly comprisesa powered crawler looped about the supporting body.
 22. The systemaccording to claim 19, wherein the underwater vehicle comprises twoconnecting assemblies, each located between the work assembly and arespective drive assembly to adjust the relative position of the workassembly and the drive assemblies.
 23. The system according to claim 22,wherein each connecting assembly comprises two tracks supported by thesustaining unit along the supporting walls; two connecting members, eachfitted to a respective track; and two first actuators, each connected toa respective connecting member and to the sustaining unit to move theconnecting member along the respective track and with respect to thesupporting wall.
 24. The system according to claim 22, wherein eachconnecting assembly comprises two articulated joints, each of whichcomprises a second actuator for adjusting the distance between arespective drive assembly and the work assembly.
 25. The systemaccording to claim 15, wherein the means for transferring the fragmentedsoil mass comprise a dredging unit comprising at least a suction portlocated between the supporting walls; and a first outlet port locateddownstream from the suction port and over the continuous elongatedmember.
 26. The system according to claim 25, wherein the dredging unitis supported entirely by the underwater vehicle.
 27. The systemaccording to claim 15, wherein the means for transferring the fragmentedsoil mass comprise a auxiliary dredging unit comprising at least aninlet port for sucking up water; and at least a second outlet portlocated at the bottom of the supporting walls to inject water betweenthe supporting walls.
 28. The system according to claim 15, wherein thefragmenting unit defines a given fragmented cross section; the width ofthe fragmented cross section being substantially equal to the distancebetween said supporting walls.
 29. The system according to claim 28,wherein the fragmenting unit is movable between a work position, inwhich it bridges the two supporting walls, and a rest position, in whichthe supporting walls enclose the continuous elongated member.
 30. Thesystem according to claim 15, wherein the work assembly comprisespushing means including at least one carriage, located between thesupporting walls; said pushing means being designed to push thecontinuous elongated member downwards.